The resistance of a device having moderate resistance (e.g. within a few orders of magnitude of 10 k ohms) can be measured with low to moderate accuracy by an ohmmeter. In an ohmmeter, current passes from one probe tip, through a first wire, through a circuit that supplies current and that measures the current flow through a resistor of the circuit, and through a second wire to a second probe tip. The accuracy of resistance measurement is affected by voltage drops along the lengths of the first and second wires due to current flows through them.
Where resistance is to be measured with high accuracy, or the electrical device has a low resistance (e.g. well below 10 ohms), a Kelvin probe system is used. In a prior Kelvin system, each probe assembly included two probe tips, one for carrying considerable current and the other for carrying substantially no current. Current flowed from the current probe tip and through a current wire, through a current source, and through the other current wire and current probe tip, to provide current to flow through contacts of the electrical device and through the device. The voltage between the two contacts of the electrical device was sensed by the separate voltage probe tips that also engaged the contacts. The voltage of each probe tip was transmitted through a separate voltage wire to a circuit that measured the voltage drop. Of course, the voltage drop for a given current through the device, enables measurement of resistance of the device. Since substantially no current passed through the voltage probe tips or voltage wires, the measurement of voltage drop between the contacts of the electrical device was highly precise. To assure that the current and voltage tips of each probe assembly securely engaged a contact of the electrical device, each probe tip was separately slideable and spring loaded. The need to provide separate current and voltage probe tips that were each slideable, resulted in high cost, less than high reliability, and difficulty in engaging small contacts. A Kelvin probe assembly which was of simple design, which was rugged, and which could be easily applied to small contacts, would be of value.
In accordance with one embodiment of the present invention, a Kelvin probe apparatus is provided for use in a Kelvin probe system that measures impedance (e.g. resistance), where the Kelvin probe apparatus is of simple, rugged, and easy-to-use design, and still allows a high current flow between its proximal and distal ends while providing an accurate indication at its distal end, of the voltage at the proximal end of the probe tip. The probe apparatus includes a probe tip mounted on a handle and having a pointed proximal end and an opposite distal end. A cable has two parallel conductors, these being voltage and current conductors. Each conductor is connected to a different terminal of a connector at the distal end of the probe assembly. Each conductor is coupled to the distal end of the probe tip. Current flows through the single pointed end of the probe tip, through the length of the probe tip to its distal end, and through the current wire to a current terminal of the connector. Substantially no current flows through the voltage conductor, so its opposite ends are at substantially the same potential, which is the potential at the distal end of the probe tip rather than the proximal end of the probe tip. However, the resistance between the proximal and distal ends of the probe tip is very low, being less than 10% of the resistance along the voltage conductor. As a result, the voltage at the distal end of the voltage conductor is very close to the voltage at the proximal end of the probe tip, thereby providing a precise indication of voltage and therefore resistance. It is noted that in some cases impedance at a particular frequency is measured, and the same probe assembly can be used for impedance measurements for a range of frequencies.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.